Michael Fallon is to warn that Ed Miliband plans to abandon Trident to ally with the SNP.
Photograph: Andy Buchanan/AFP/Getty Images

The Tories will issue their strongest warning about the dangers of a post-election pact between Labour and the Scottish National party when they accuse Ed Miliband of preparing to abandon Britain’s nuclear weapons as the price of securing power.

In an echo of Margaret Thatcher’s attacks on Labour’s defence policy in the 1980s, the defence secretary, Michael Fallon, will claim on Thursday that Miliband would “barter away our nuclear deterrent” and that the Labour leader would bow to the SNP’s “childlike worldview”.

Some Tories also criticised Fallon after he launched a highly personal attack on Miliband, saying that he could not be trusted with the nation’s defences after he “stabbed his own brother in the back to become Labour leader”. In a Times article Fallon wrote: “Now he is willing to stab the United Kingdom in the back to become prime minister.”

Tim Montgomerie, the founder of the ConservativeHome website, tweeted: “Embarrassing: way too personal from Michael Fallon against Ed Miliband.”

The defence secretary will say: “There will be a clear choice between a Conservative government that will put our national security first. Or Ed Miliband, a man so desperate for power he is ready to barter away our nuclear deterrent in a backroom deal with the SNP.”

The Tories’ decision to highlight a threat to Trident on Thursday is their latest high-stakes bid to boost support in England by warning of the dangers of a post-election deal between Labour and the SNP. Opinion polls, which suggest that the SNP could win more than 40 of Scotland’s 59 seats, indicate that the only path to Downing Street for Labour may involve a deal with the SNP. Miliband has ruled out a coalition with the SNP but has declined to rule out a more informal arrangement.

Fallon will say that Miliband will have to abandon the renewal of Trident after Nicola Sturgeon, the SNP first minister, made the scrapping of nuclear weapons a red line in any coalition negotiations. The defence secretary will say: “The SNP’s childlike worldview would sacrifice the long-term security of the UK and play into the hands of our enemies. The nuclear deterrent protects all of Britain and the SNP represents a separatist threat that would dangerously weaken our collective defence.

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“Nicola Sturgeon has offered to make Ed Miliband prime minister if he meets her shopping list of more debt, unlimited welfare and junking our nuclear deterrent. It is a sign of Ed Miliband’s weakness that he has failed to rule out a deal. Voters can only conclude that he would be prepared to trade Britain’s national security just to get his hands on the keys to Downing Street.”

Fallon will also announce in the speech that a future Conservative government would replace the current Vanguard nuclear submarine fleet with four new vessels to guarantee Britain’s “continuous at-sea deterrence” (CASD).

Labour last night dismissed the speech as scaremongering by the Tories, who were unnerved after Labour secured a two-point lead over them in a YouGov/Sun poll, by 35% to 33%. The party pointed out that Miliband gave a simple “no” answer when Jeremy Paxman asked him in the recent Channel 4/Sky News Q&A whether he would agree to an SNP demand not to renew Trident.

Coaker said: “As Ed Miliband has already made crystal clear, national security is not a matter for negotiation. We support renewal of Trident along with a renewed focus on multilateral disarmament.

“Labour is committed to maintain a minimum, credible, independent nuclear deterrent, delivered through a continuous-at-sea deterrent. This is not up for negotiation with any party. The Tories are resorting to the language of smear. They are increasingly desperate as their campaign lurches on in confusion and chaos.”

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Labour has said it is looking at the possibility of building three submarines to reduce the £25bn costs of replacing Trident when the “maingate” decision is taken in 2016. But it will only do so if CASD can be maintained.

Sturgeon indicated in a Guardian interview last month that the SNP would not place the scrapping of Trident on the table in any negotiations after the election. The Scottish first minister said the SNP would simply vote against its replacement when the maingate vote is held in the Commons.

But in the Scottish leaders’ debate on the BBC on Wednesday night, Sturgeon said: “Is Trident a red line? Well here’s my answer: you’d better believe Trident is a red line.”